The present invention relates to borehole drilling, and particularly to determining and controlling the drilling direction of a borehole between specified borehole entry and exit locations.
From time to time, pipelines, wirelines (e.g., power transmission lines, fiber optic cables, communication lines, etc.), or other such facilities must be installed in locations where they must cross an obstacle (e.g., structures such as railroad tracks, highways, rivers, etc.) because the ground above the borehole is difficult or impossible to access, and/or the obstacle cannot be removed or relocated. Thus, various drilling techniques have been used to install pipelines and/or wirelines underneath these obstacles.
Specifically, a drilling apparatus is employed to tunnel a borehole underneath the obstacle from a specified entry point to a specified exit location in order to safely traverse the existing obstacle. After the tunnel is completed, the borehole may receive a casing that accommodates the pipeline or cable(s). Importantly, the borehole tunnel must maintain a certain clearance, or distance, from the preexisting obstacle so that the borehole tunnel does not interfere with, or otherwise damage, the preexisting structure. In other words, the drilling process must be carefully controlled so that the drill bit and the associated drilling means follows a prescribed drilling right-of-way (i.e., route) interconnecting a predetermined tunnel entry point and a predetermined tunnel exit point. The borehole must therefore remain within this drilling right-of-way as it passes under the obstacle and emerges (at or very near the predetermined exit point) on the other side of the obstacle.
In one approach that has been considered, a directional (e.g., horizontal) borehole can be drilled by a conventional directional drilling system. This conventional system may include a drilling tool having a motor disposed down-hole and operatively connected to a bent drill housing. The bent housing is, in turn, mounted on the lower end of a string of drill rods or tubing. The drill bit (tool) is rotatably mounted on the lower end of the bent housing and is driven by a down-hole motor. The drilling motor is typically powered by a viscous fluid that is directed down the inside of the drill rods or tubing. The bent housing has an upper section and a lower section. One end of the bent housing is connected to the down-hole motor housing, while the distal end is connected to a drill rod, typically approximately 1.5 meters long. The drill rod extends away from the housing at a small angle (e.g., 2-3 degrees from the borehole axis) and is connected to the drill bit. The bent housing causes the drill to swing at an arc when it rotates; and thus, the rotational motion must be controlled from the surface. The majority of the drilling power is provided by the down-hole motor. To provide straight ahead drilling, the bent housing, the motor housing and the drill rods are rotated continuously by a motor on the drill floor at the Earth's surface. To change the direction of drilling, the roll angle of the drill rods is held fixed at the surface, while the drill bit is powered solely by the stator in the hydraulic down-hole motor. It is imperative that the roll angle orientation of the bent lower section be known and controllable. The roll angle orientation of the bent section is known relative to the hydraulic down-hole motor (since the two elements are coupled together). In addition, a down-hole electronic “measurement while drilling” (MWD) package is fastened inside a non-magnetic section of drill rod just above the motor. Accordingly, the roll angle orientation, as well as the inclination and the azimuth of the borehole, are communicated to an up-hole control system by way of electrical signal wires or other communication system to the driller. The driller uses these data to adjust the drilling apparatus to drill straight ahead or along the specified curved path.
Accordingly, one of the drawbacks to this approach relates to the cost and complexity of placing and operating the down-hole system. The down-hole motor, the hydraulic drive system, and the electrical signaling infrastructure required to control the orientation of the drilling tool face can be expensive. Moreover, there is no guarantee that the above described control system provides the needed accuracy.
Another system utilizes a system of dual coaxial drilling rods which includes an inner set of rods whose sole function is to power the drilling bit. There is no down-hole drilling motor, all drilling power comes from rotating the inner drill rod at the surface. A second outer coaxial set of drills rods, independently rotatable from the inner set of drill rods, includes a ‘bent sub’ near the drill bit. The roll angle orientation of the “bent” sub controls the direction of drilling as with the down-hole motor system described above. With this system, rotation of the outer set of drill rods has the sole function of controlling the roll angle orientation of the bent sub near the drill bit. For straight ahead drilling, the outer drill rods, with the attached bent sub, are rotated at the same time as the inner rods, fastened to the drill bit, and rotated. To change the direction of drilling, the outer drill rod string is held fixed at the surface, thereby holding the roll angle of the bent sub steady. A difficulty in determining the bent sub orientation is that the many drill rod connections are usually screw connections with the relative orientation between the drill rods being unknown.
In such systems the current drill bit location and “bent” sub orientation are typically determined using a “walk over” system. These systems often consist of a simple down-hole electronic transmitter package attached to the outer set of drill rods near the drill bit which transmits an electromagnetic signal with encoded data to a person carrying a receiver who walks back and forth directly above the transmitter. While simple, these systems have limited range vertically and cannot be used in many circumstance where accessibility to the required surface location to make the measurements is not possible
What is needed, therefore, is a method and apparatus that obviates the difficulties identified above. Specifically, a self-contained in the down-hole method and apparatus are needed for controlling and guiding the drilling of a borehole with increased precision and accuracy. This package must also allow simultaneous determination of drill face orientation direction as well as drilling azimuth and inclination.